Sewage Sludge Ash Research Articles

Synergistic recycling of MSWI fly ash and incinerated sewage sludge ash into low-carbon binders

This study proposed a novel method for synergetic recycling of municipal solid waste incineration fly ash (IFA) and incinerated sewage sludge ash (ISSA) to produce low-carbon binders. The hydration mechanism, physical properties, and environmental risks associated with the designed binders were thoroughly evaluated. The results revealed a compositional complementary effect between IFA and ISSA, with ettringite being identified as the main crystal product resulting from the reaction between Ca2+ and SO42- in IFA and reactive aluminates in ISSA. The chlorides in raw IFA were effectively immobilized by the formation of various Cl-containing products, and all potentially toxic elements were immobilized in the binder system, exhibiting low leachability. Besides, CaO was found to be a highly efficient activator, providing additional reactive Ca2+ for the formation of more abundant and larger ettringite crystals, which consequently led to the high compressive strength (> 40 MPa) of the designed binders. Overall, this study provided a feasible approach for the upcycling of IFA and ISSA, contributing to the realization of “Zero-waste city” and the advancement of sustainable construction materials.

Effects of phosphorus pentoxide on viscosity-temperature characteristics of sewage sludge ash slags under mild reducing atmosphere

Gasification is one of the promising ways for resourceful utilization of municipal sewage sludge (MSS). For the application of MSS to slagging gasification technology, the effects of phosphorus pentoxide (P2O5) content on slag viscosity-temperature characteristics needs to be investigated. In this study, two series of representative slags containing 0–20 wt% of P2O5 with Al2O3/CaO ratio of 2:8 and 8:2 were prepared synthetically. The viscosity-temperature curves of samples were measured, and the structures and the crystalline behavior of slags were characterized to explore the role of P2O5 in the slags. The results show that the addition of phosphorus slightly increased the high-temperature viscosity of Ca-rich slags but significantly raised their temperatures of critical viscosity (TCVs). When P2O5 content exceeded 10 %, the slag type changed from glassy to crystalline. As for the Al-rich slags, although their viscosity and TCVs decreased significantly with increasing phosphorus content, it was not suitable for use in the slagging gasifier due to their high viscosity. According to Raman and NMR spectra, the addition of phosphorus induced the repolymerization of silica-oxygen tetrahedra in both kinds of slags. However, partial Al3+ ions in the Al-rich slag shifted from tetra- to penta-coordination and extensive [AlPO4]-like units were formed, which weakened the effect of silicon-oxygen network repolymerization in increasing the slag viscosity. SEM-EDS map-scanning results revealed that the precipitation of apatite crystals was an important factor of the rapid increase in viscosity of the Ca-rich phosphorus-containing slag during cooling. The elevated phosphorus content, on the other hand, changed the type of crystals precipitated in the Al-rich slag from large-particle sillimanite to small-particle berlinite, which further moderated the increase in viscosity.

Laboratory and field investigations of alkali-silica reaction prevention by supplementary cementitious materials: Influence of the free alkali loading

Performance tests are often used to evaluate the effect of supplementary cementitious materials (SCMs) on alkali-silica reaction (ASR). However, these tests may be quite demanding, thus other approaches may be more efficient to identify the materials that are worthwhile testing further. This paper presents ASR expansion data obtained from outdoor exposure (2.5 years) and three accelerated laboratory tests: ASTM C1567, TI-B 51, and RILEM AAR-10. A reactive sand containing porous flint, which caused damage in structures after only a few years, was tested with Portland cement and a wide range of SCMs: fly ash, two biomass ashes, two calcined clays, sewage sludge ash, crushed brick, and glass beads. Expansion data were compared with free alkali loadings determined on companion paste samples. It was observed that no expansion occurred below around 2.4 kg/m3 free Na2Oeq in AAR-10 and outdoors (so far). Conversely, no clear trend was found with ASMT C1567 and TI-B 51 results, probably because the outcomes of these immersion tests depend on the transport properties of the specimens and interactions with the surrounding solutions. Since expansions in AAR-10 and outdoors correlate, the free alkali loading seems a promising parameter for screening how SCMs influence the risk of deleterious ASR.

Ex Situ Stabilization/Solidification Approaches of Marine Sediments Using Green Cement Admixtures.

The routine dredging of waterways produces huge volumes of sediments. Handling contaminated dredged sediments poses significant and diverse challenges around the world. In recent years, novel and sustainable ex situ remediation technologies for contaminated sediments have been developed and applied. This review article focuses on cement-based binders in stabilizing contaminants through the stabilization/solidification (S/S) technique and the utilization of contaminated sediments as a resource. Through S/S techniques, heavy metals can be solidified and stabilized in dense and durable solid matrices, reducing their permeability and restricting their release into the environment. Industrial by-products like red mud (RM), soda residue (SR), pulverized fly ash (PFA), and alkaline granulated blast furnace slag (GGBS) can immobilize heavy metal ions such as lead, zinc, cadmium, copper, and chromium by precipitation. However, in a strong alkali environment, certain heavy metal ions might dissolve again. To address this, immobilization in low pH media can be achieved using materials like GGBS, metakaolin (MK), and incinerated sewage sludge ash (ISSA). Additionally, heavy metals can be also immobilized through the formation of silicate gels and ettringites during pozzolanic reactions by mechanisms such as adsorption, ion exchanges, and encapsulation. It is foreseeable that, in the future, the scientific community will increasingly turn towards multidisciplinary studies on novel materials, also after an evaluation of the effects on long-term heavy metal stabilization.

Synergistic recovery of aluminum phosphorus from incinerated sewage sludge ash via acid and alkaline treatment

The “phosphorus (P) crisis” has increased interest in improving P recovery methods and their efficiency. Incineration of sewage sludge ash is a recognized method for efficient P recovery, but challenges persist because of low recovery efficiency using thermochemical or biological methods and the issue of heavy-metal contaminants. Traditional wet-chemical processes, which yield calcium phosphate as the main product, are complex and costly. In contrast, aluminum (Al) phosphate (AlP) not only serves as a slow-release phosphate fertilizer but also has diverse high-value industrial applications, such as its use in battery coatings. To address these challenges, we developed a novel recovery route based on AlP extraction from Al-rich incinerated sewage sludge ash via acid leaching and alkaline precipitation. Our findings indicate that nitric acid (HNO3) achieves the highest Al leaching efficiency. The leaching kinetics of PO43− and metals follow second-order kinetics. Furthermore, pH adjustment with NaOH showed that the P precipitation efficiency from oxalic acid leachate was lower than that from hydrochloric acid, sulfuric acid (H2SO4), and HNO3 (88.0%–99.8%) because of buffering effects. These results underscore the potential of Al-rich ash for effective P recovery and considerably broaden the range of applications for P products.

Carbide slag and sodium metasilicate synergistic activation of sludge ash-based alkali-activated materials: Towards a cleaner activation approach

Traditional activators such as sodium hydroxide and sodium silicate are commonly used in the preparation of alkali-activated materials; however, their significant environmental impact, high cost, and operational risks limit their sustainable use in treating solid waste. This study explores the innovative use of carbide slag (CS) and sodium metasilicate (NS) as alternative activators in the production of sewage sludge ash-based alkali-activated materials (SSAM) with the aim of reducing the carbon footprint of the preparation process. The results demonstrate that CS effectively activates the sewage sludge ash, enhancing the compressive strength of the SSAM to 40 MPa after curing for 28 d. When used in conjunction with NS, it synergistically improves the mechanical properties. Furthermore, the microstructure and phase composition of the SSAM are characterized. Increasing the quantities of CS and NS accelerates the dissolution of the precursor materials, promoting the formation of a higher quantity of hydration products. This significantly reduces the number of voids and defects within the samples, further enhancing the densification of the microstructure. Environmental assessments reveal that CS and NS offer substantial sustainability benefits, confirming the feasibility of activating SSAM using these materials. This approach provides a less energy-intensive and more environmentally friendly alternative to conventional activation methods and presents an effective strategy for managing large volumes of sewage sludge ash and CS.

Performance improvement of glass-based lightweight aggregates through thermodynamic modelling design and lightweight mortar validation

To reduce environmental impact of sintered lightweight aggregates (LWAs), in this study, novel glass-based LWAs were produced from municipal solid wastes at 900 °C. The mix was designed by thermodynamic modelling to optimize the phase composition, liquid phase properties, and microstructure of the LWAs. Results demonstrated that the inclusion of incinerated sewage sludge ash (ISSA) increased the viscosity of the liquid phase, resulting in a refined pore structure characterized by smaller, more isolated pores, and thicker pore walls. Additionally, ISSA facilitated the formation of various crystalline phases (e.g. wollastonite) to stabilize the pore structure, thereby enhancing densification and matrix strength. Leaching behaviour tests confirmed the suitability of glass-based LWAs for environmentally safe lightweight products. Lightweight mortars incorporating developed LWAs exhibited a high 28-day compressive strength (up to 57.9 MPa) with a low density of 1688 kg/m3 and a low thermal conductivity of 1.0 W/(m·K). The energy simulation revealed substantial reductions in energy consumption when the lightweight mortars were utilized in the building compared with the reference building using normal-weight C30 concrete, underscoring its promising role in conserving energy and reducing carbon emissions.

Binary solid wastes derived clinker: Raw feed system design validation and thermodynamic simulation investigation

Solid waste derived clinker design methodology was established based on thermodynamic simulation and experiment validation via the recycling of incinerated sewage sludge ash (ISSA) and recycled concrete fine (RCF). Compared with 1000 °C and 1100 °C, 1200 °C was the optimum temperature for C2S-rich clinker synthesis due to the highest simulated belite content after sintering. A utilization rate of 95 % was achieved, accomplishing the maximization of solid waste recycling. Due to the CaCO3 phase formation by the reaction of high-content belite (46.8 %) with CO2, carbonated 1200ARS eco-cement showed higher compressive strength than OPC and other eco-cements. The lowest porosity (18.1 %) with dense microstructure was also obtained by carbonated 1200ARS. Benefiting from the high CO2 reactivity of belite, 1200ARS displayed the optimum CO2 sequestration capacity with a higher carbonation degree (30.1 %) than other batches, providing a promising direction for future development of the low-carbon cement industry to achieve environmental sustainability.

Extraction of phosphorus from sewage sludge ash by electrodialysis combined with wet-chemical extraction

ABSTRACT Phosphorus (P) recovery from sewage sludge ash (SSA) is considered to be an effective method for P recovery. In this work, P extraction and the removal of heavy metals were realized by electrodialysis. Low-cost, easily available, and environmentally friendly plant extracts were applied as suspension to reduce the inevitable secondary pollution. And the feasibility of using plant extracts was analysed by comparing with using deionized water (DI) and oxalic acid (OA) solution. When SSA was suspended in different solutions (DI, OA, and three plant extracts – Hovenia acerba (HA), Saponin (SA) and Portulaca oleracea (PO)), the effects of reaction time and plant extract concentration on P extraction and heavy metal separation of SSA under ED treatment were compared. After the process of electrodialysis, compared to other experimental groups, electrodialysis with plant extracts obtained more P released from SSA, but less P migrated to the anode chamber. However, when SSA was suspended in PO at a concentration of 80 g/L, the proportion of P transferred from SSA to the anode chamber can still reached 37.86%. In addition, the use of plant extracts as suspension had a positive effect on the removal of heavy metals, but its effect was lower than that of the oxalic acid-treated experimental group. The results indicated that the use of plant extracts for wet-chemical extraction combined with electrodialysis promoted the removal of heavy metals and the extraction of P from SSA, which is a feasible option.

Circular economy approach: Nutrient recovery and economical struvite production from wastewater sources by using modified biochars

The enrichment of phosphorus (P) and nitrogen (N) in aquatic systems can cause eutrophication. Moreover, P rocks may become exhausted in the next 100 years. A slow-release fertilizer called struvite (MgNH4PO4.6H2O) can reduce surface runoff. However, the high cost of raw material or chemicals is a bottleneck in their economical production. Therefore, incinerated sewage sludge ash, food wastewater, and bittern were combined as the sources of P, N, and Mg, respectively. Sawdust biochar was used to enhance the adsorptive recovery of nutrients. First, recovery kinetics was studied by comparing bittern-impregnated biochar (BtB) with the Mg-impregnated biochar (MgB). Subsequently, the synergistic physical and chemical interactions were observed for P and N recovery. Almost complete PO43–P recoveries were achieved within 10 min for both biochars. However, NH4+-N recovery was stable after 2 h, with 26% recovery by MgB and 20% recovery by BtB. Biochars activated with steam (steam-activated biochar) and KOH (KOH-activated biochar) gave superior activities to those of unactivated biochars and activated carbon (AC) nutrient recovery and struvite purity. Moreover, the activated biochars showed a lower risk of surface runoff, similar to that of AC. Therefore, activated biochars can be used as an alternative to AC for economical struvite production from a combination of wastewater sources.

Hydrothermally Stimulated Molecular Interfaces for Augmented Electron Delocalization in Wet-Chemical Phosphorus Recovery from Incineration Ash of Sewage Sludge.

Wet-chemically recovering phosphorus (P) from sewage sludge incineration ash (SSIA) has already become a global initiative to address P deficit, but effectively isolating P from these accompanying metals (AMs) through adsorption in a SSIA-derived extract remains elusive. Here, we devised a hydrothermal stimulus-motivated thermodynamic and kinetic enhancement to gain anionic ethylenediaminetetraacetic acid (EDTA) molecular interfaces for AM enclosure to resolve this conundrum. A new dosage rule based on the EDTA coordination ratio with AMs was established for the first time. Upon hydrothermal extraction at 140 °C for 1 h, the P extraction efficiency reached 96.7% or higher for these obtained SSIA samples, and then exceptional P sequestration from these EDTA-chelated AMs was realized by the peculiar lanthanum (La)-based nanoadsorbent (having 188.86 mg P/g adsorbent at pH ∼ 3.0). Relevant theoretical calculations unraveled that these delocalized electrons of tetravalent EDTA molecules boosted the enclosure of liberated AMs, thereby entailing a substantially increased negative adsorption energy (-408.7 kcal/mol) of P in the form of H2PO4- through intruding lattice-edged carbonates to coordinate La with monodentate mononuclear over LaCO5(1 0 1). This work highlights the prospect of molecular adaptation of these common extractants in wet-chemical P recovery from various P-included wastes, further sustaining global P circularity.

Effect of silica fume on the efflorescence, strength, and micro-properties of one-part geopolymer incorporating sewage sludge ash

The application of sewage sludge ash (SSA) in alkali activated cementitious material, i.e., geopolymer, was one of the effective measures to dispose of and recycle the sewage sludge. However, the geopolymer efflorescence could be aggravated due to elevated alkali content, especially for one-part geopolymer, where the dissolution reaction of solid alkali activator was less sufficient. This study focused on the efflorescence of GGBS-SSA based one-part geopolymer, as compared to two-part geopolymer, and the effect of silica fume on the efflorescence, strength and micro-properties of the one-part geopolymer. The results showed that the extent of efflorescence crystallization and the concentration of leached Na+ ions in the one-part geopolymer were more severe than the two-part geopolymer, due to the lower degree of geopolymerization reaction and the heterogeneous dense microstructure of the one-part geopolymer. After adding silica fume to the one-part geopolymer, the extent of efflorescence crystallization and the concentration of leached Na+ ions reduced significantly, and the compressive strength increased from 41.6 MPa to 57 MPa when the silica fume content was 10 %, but then decreased slightly. Adding the silica fume reduced the release rate of reaction heat, which helped to increase the solubility of solid alkali activator and improve the geopolymerization reaction, thus significantly reducing the content of mobile alkali ions and enhancing the compressive strength. Moreover, the silica fume with tiny spherical particle had better ball effect and micro-aggregate filling effect, which refined the pore size and reduced the connectivity of pore network, significantly reducing the diffusion rate and leaching of mobile alkali ions.

Investigation on solidified/stabilized behavior of marine soil slurry by lime-activated incinerated sewage sludge ash-ground granulated blast furnace slag under multifactor conditions

This study aims to evaluate the possibility of reusing treated marine clayey soils by stabilization/solidification (S/S) technology as geomaterial in reclamation projects from the aspects of engineering strength, chemical modification and environmental risk assessment. The lime-activated incinerated sewage sludge ash (ISSA) together with ground granulated blast furnace slag (GGBS) was employed as the binder. The multi-controlling factors including water content, curing time, salinity, and chemical compositions of mixing solution were taken account to identify the S/S treated Hong Kong marine deposit (HKMD) slurry based on the strength tests, pH measurement, thermo-gravimetric (TG) analysis, X-ray diffractometer (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy coupled with energy-dispersive spectrometry (SEM-EDS) and toxicity characteristic leaching procedure (TCLP) tests, etc. The results show that the S/S treatment using lime-activated ISSA-GGBS can effectively enhance the strength of marine soil at initial water content of 110% and 200%. The water content and curing time have a significant impact on the S/S treated HKMD. The pH of treated soils is higher than 11.1, which proves an alkaline environment for the reactions in the treated soil. A special case is the treated HKMD at 200% water content hydrated by MgCl2 solution, which has a low pH of 10.23 and maintains a slurry state. Based on the TCLP results, the leaching concentration of heavy metals from S/S treated HKMD is environmentally safe and meets Hong Kong standard for reusing treated soil with a low level of <0.2 mg/L. The content of main products such as calcium/magnesium silicate hydrate, ettringite or Friedel's salt depends on the chemical additions (e.g. distilled water, seawater, NaCl and Na2SO4). The products in the specimens mixed with MgCl2 solutions are mainly composed of Mg(OH)2, M-S-H and MgCO3, which is distinct with the neoformations in the other cases. Therefore, this study proves that the S/S treated soil slurry could be reused as geomaterials in reclamation projects, and the S/S process is greatly affected by water content, curing time and solution compositions, etc.

Phosphorus speciation in sewage sludge and their ashes after incineration as a function of treatment processes.

Phosphorus (P) is a key component in agricultural fertilizers, but it is also a scarce resource, why its recycling has been thoroughly investigated and one promising resources is sewage sludge. Because of stricter regulations in terms of sludge disposal, thermal treatment (e.g. incineration) has become an attractive option. The incineration process alters the chemical speciation of P in favour to calcium-associated (apatite, apatite phosphorus (AP)) species, which is preferred for P recovery. In order to achieve qualitatively transformation, it is important to identify limiting or promoting factors. This study reports on the impact of iron, aluminium and calcium on the transformation of iron- and aluminium-phosphate (NAIP) to AP species, assessed by studying sludge and ash from 10 municipal wastewater treatment plants in Sweden. The effect of iron and aluminium added in the treatment processes was also evaluated. The obtained results show that high calcium concentration favours formation of AP species in both sludge and ashes, whereas high concentration of iron and aluminium favours formation of NAIP species in the sludge. The transformation from NAIP to AP species is hampered by aluminium, irrespectively of its origin, whereas no such correlations could be seen for iron. Therefore, in order to enable efficient P recovery from sewage sludge ash, the amount of aluminium added in the treatment process, as well as its concentration in influent streams to the treatment plants, must be limited.

Migration of trace elements and radioisotopes to various fractions of solid wastes generated as a result of the sewage sludge incineration process

The research was aimed at providing new knowledge in the field of chemical characteristics of solid waste generated in the process of combustion of sewage sludge in fluidized bed furnaces. The research material consisted of disposed fluidized beds (DFB), sewage sludge ash (SSA) and air pollution control residues (APC) from three Polish installations for the thermal treatment of sewage sludge. Natural radionuclides as well as anthropogenic isotope 137Cs were determined in the tested materials and the migration of a wide spectrum of trace elements to various waste fractions generated in the process of sewage sludge combustion was examined. It was observed that both radioisotopes and most of the trace elements determined accumulate in SSA and DFB, while the APC fraction contains a much smaller amount of them. The exceptions are mercury and selenium, whose volatile compounds migrate to the exhaust gas dedusting system and accumulate in the APC fraction (up to 40 mg/kg and 13 mg/kg, respectively). A potential threat from the 226Ra isotope in SSA is identified in the context of the management of this waste in the production of building materials because the typical activity of 226Ra in SSA collected from areas with very low Ra content in natural environment exceeds 1.5–6 times the activity of this isotope in conventional cement mixtures. When managing SSA and DFB, special attention should be paid to the content of metalloids such as As, B and Se, due to the high content of mobile forms of these elements in the mentioned materials.

The Effect of Renewable Phosphorus Biofertilizers on Selected Wheat Grain Quality Parameters

Recycling and reusing phosphorus in agriculture can reduce the consumption of natural phosphorus resources, which are continuing to shrink. Phosphorus fertilizers made from renewable raw materials (sewage sludge ash, animal bones, dried animal blood) and activated with phosphorus solubilizing microorganisms (Bacillus megaterium, Acidithiobacillus ferrooxidans) offer an alternative to conventional fertilizers. These products should meet consumer and environmental safety standards. In this paper, based on field experiments conducted in northeast Poland, the effects of waste-derived biofertilizers on selected parameters of wheat yield quality are discussed. The study focuses on the technological properties of the grain (hectoliter weight, hardness index, Zeleny index, starch, wet gluten, and protein content), the content of proteogenic amino acids, macro- and micronutrients, and selected toxic elements in the grain. The quality parameters of wheat grain were not affected by the tested biofertilizers applied in P doses up to 35.2 kg ha−1, nor by conventional fertilizers.

Comparative Analysis of Laboratory-Made and Industrial-Made Sewage Sludge Ash: Implications for Effective Management Strategy Development.

In the pursuit of environmentally and economically sustainable sewage sludge ash (SSA) management methods, researchers often employ laboratory-made SSA (L-SSA) as a substitute for industrial-made SSA (I-SSA) produced in fluidized bed furnaces. To check whether L-SSA is a material that imitates I-SSA well, the fractionation of metals whose presence is a significant problem during SSA management was performed. In addition, the grain distribution, specific surface area, and textural properties of the tested materials were examined. Differences in total Pb and Hg content and mobility of Cu, Ni, Mn, and Zn were observed between I-SSA and L-SSA. Larger particle sizes of L-SSA compared to I-SSA were confirmed, while comparable textural properties and specific surface area of both types of materials were maintained. Based on the results, it was concluded that L-SSA is chemically different compared to I-SSA, and that L-SSA should not be used as a reference in research focused on the design of SSA management methods. Moreover, fractionation of metals was performed in disposed fluidized beds (FBs), which are diverted to non-hazardous waste landfills without prior analysis. It has been proven that studied metals are present in FBs as abundantly as in SSA, while Cu, Mn, and Ni may show higher mobility than in I-SSA.

Optimization of phosphorus wet acid extraction from sewage sludge ashes: Detailed process insight via multi-variate statistical techniques

To tackle the issue of phosphorus (P) supply, Sewage Sludge Ash (SSA) is a promising secondary source. The goal of this research is the optimization of the wet P extraction process in the view of providing valuable insights for the process scale-up. While most of the studies relied on the “one-variable-changed-at-a-time” approach, this work applied a statistical optimization model based on the Design of Experiments technique for investigating the influence of most relevant operating conditions of extraction process on Phosphorus Extraction Efficiency (PEE) with two types of SSA characterized by different levels of alkalinity. Experimental results highlighted the importance of ash characteristics, making some SSA not suitable for cost-effective phosphorus recovery due to the high acid dosage required in extraction process. Type of acid and contact time become significant factors only for Ca-rich ashes, especially when using H2SO4 as extracting agent. Optimal operating conditions (CH2SO4 = 1.3 N, L/S = 20, t = 2 h and CHCl = 2.0 N, L/S = 10, t = 2 h for Ash#1 and CH2SO4/HCl = 0.8 N, L/S = 10 for Ash#2) resulting in high PEE (80–94%) were identified for extraction performed with H2SO4 and HCl as a function of ash characteristics.

Recycling of incinerated sewage sludge ash and waste glass powder in alkali-activated slag for sewer rehabilitation

A new era has dawned in the manufacturing of cement-free binders with appropriate mechanical strengths and durability to combat CO2 emissions. However, the assessment of their performance in extreme conditions is ongoing. Here, we attempted to use incinerated sewage sludge ash (ISSA), a waste product of sewage sludge incineration that contains limited amounts of heavy metals, along with waste glass powder (GP) and ground granulated blast furnace slag (GGBS), as precursors to produce cement-free binders through alkali-activation. The alkali-activated materials (AAMs) were then subjected to an intensified sewage corrosion test for 6 months. The aim was to utilize the heavy metals in the ISSA as biocides to resist the biogenic acid attack on the AAMs. The experimental results indicated that superior performance was achieved by using a ternary binder prepared with ISSA, GP, and GGBS under biogenic acid simulation. Such enhanced durability can be attributed to the low Ca content in the resulting alkali-activated gels, which also reduced the grain size of gypsum formed and prevented expansion deterioration. Furthermore, the slow release of heavy metals from the AAMs prepared with the ISSA, evidenced by the leaching test results, was able to inhibit microbial growth.

Multiplicative effects of co-incineration of sewage sludge and wasted oyster shells in air, CO2, and steam atmospheres on P speciation and bioavailability and metal ecotoxicity

The study was aimed at the reutilization of oyster shells (OS) as a calcium-based additive to turn sewage sludge ash into phosphate fertilizer, employing the oxygen-enriched combustion technology. This process transformed all non-apatite inorganic phosphorus into apatite phosphorus with the incorporation of a 7.5% Ca-based additive. OS demonstrated comparable effectiveness to CaO in catalyzing this transformation in the oxygen-steam atmosphere. The amount of OS added and the atmosphere type significantly influenced the P bioavailability of sewage sludge ash. The oxygen-steam atmosphere, with its increased water vapor concentration, facilitated the decomposition of CaCO3 and initiated reactions with calcium phosphate to form hydroxyapatite, thus substantially enhancing P bioavailability. The oxygen-steam atmosphere increased not only metal volatilization but also the ecological toxicity of residual metals, particularly Pb, mainly in the form of F2. Excessive OS addition appeared to elevate the toxicity of Cr. Therefore, additional strategies for metal removal or passivation are needed to improve the safety of sewage sludge ash with high metal content.